ULTRASOUND EVALUATION OF THE CAROTID ARTERIES

PITFALLS IN CAROTID
ULTRASOUND
DISCLOSURES
• Educational consultant for Philips HealthCare
Leslie M. Scoutt, MD, FACR
Professor of Diagnostic Radiology & Surgery
Chief, Ultrasound Section
Medical Director, Non-Invasive Vascular Lab
Yale University School of Medicine
OUTLINE
• How to avoid technical pitfalls
• How to avoid pitfalls in interpretation
– Anatomic
– Physiologic
• How to differentiate a nearly occlusive
stenosis from a complete occlusion
TECHNIQUE: Pitfalls
• Incorrect Doppler angle:
SPECTRAL DOPPLER
TECHNIQUE
• Angle between 450 and 600
– keep angle constant on f/u exams and for all vessels
• Make several measurements (usu 3)
• Record highest
– PSV
– PSVR
– +/- EDV
TECHNIQUE: Pitfalls
• Incorrect measurement of Doppler angle
45° - 60°
use same angle for serial studies
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PITFALLS: Technique
• Incorrect placement of Doppler sample volume
– should be in center of vessel or jet
TECHNIQUE: Pitfalls
• Measurement of Doppler angle
PITFALLS: Technique
• Incorrect placement of Doppler sample volume
– if measure PSV in distal CCA too close to bulb
where the vessel diameter has begun to widen,
PSV will be falsely low
– may result in falsely elevated PSVR
TECHNIQUE: Pitfalls
• Sample volume size
– ? parallel to vessel wall
– ? parallel to jet of blood
or residual lumen
DOPPLER CRITERIA
• Range of absolute numbers and ratios for any
given % stenosis
– laboratory dependent
• Can not accurately differentiate carotid stenoses
@ 10% increments
• Tend to overestimate carotid stenoses
Sabeti, Radiology: 2004
DOPPLER CRITERIA
• More accurate for detecting high grade stenoses
(70-99%)
• Less accurate for low grade stenoses (< 50%)
• Only validated for the ICA
Sabeti, Radiology: 2004
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DOPPLER CRITERIA
• Whatever criteria you choose,
– the closer you are to the discriminatory value, the
more likely you are to be wrong
– the farther away you are from the discriminatory
value, the more likely you are to be right
• Consider correlative imaging if close to
discriminatory thresholds
PITFALLS: Calcified Plaque
• Shadowing may obscure vessel lumen
– jet will persist for ~ 1 cm
PITFALLS: Anatomic & Physiologic
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•
•
•
•
Calcified plaque
Cardiac arrhythmias
High and low cardiac output states
Multiple areas of plaque, tandem lesions
Long segment stenoses
PITFALLS: Cardiac Arrhythmia
•  HR results in  PSV,  EDV
•  HR results in  PSV,  EDV
• Therefore, if plaque shadows for > 1 cm, may
miss a stenosis
• If shadowing obscures
lumen for < 1 cm,
unlikely to miss a
significant stenosis
PITFALLS: Tachycardia
• Underestimates PSV
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PITFALLS: Cardiac Arrhythmia
• Measure PSV consistently in patients with
arrhythmias
– after most normal appearing R-R interval
– avoid PVC or beat following compensatory pause
PITFALLS: Cardiac Arrhythmia
• Measure PSV consistently in patients with
arrhythmias
– if no “normal” appearing beat, choose highest
(or lowest) PSV
PITFALLS: Cardiac Output
PITFALLS: Abnl Cardiac Output
• If PSV in CCA is > 100 cm/s or < 60 cm/s
• High output states → PSV in CCA > 100 cm/s
– absolute PSV likely not as accurate
– put more emphasis on PSVR, grey scale and
color Doppler imaging
PITFALLS: Cardiac Output
• Hyperdynamic state
• PSV will overestimate % stenosis
– hypertension
– hyperdynamic state
– aortic regurgitation
– thyrotoxicosis
PITFALLS: Abnl Cardiac Output
• Low output states → PSV in CCA < 60 cm/s
– ↓ ejection fraction

cardiomyopathies, LV dysfunction, LV aneurysm
– aortic stenosis
– hypotension
– thoracic aortic aneurysm
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PITFALLS: Cardiac Output
• PSV in CCA = 35 cm/s
• When PSV in ICA
reaches 230 cm/s, PSVR
will be > 6.5
• Relying on PSV will
result in underestimation
of ICA stenosis
DOPPLER CRITERIA
• Whatever criteria you use:
– Doppler criteria should be concordant
– ALWAYS correlate with grey scale/color Doppler
appearance and waveform
• Explain any discordance
EF = 15%
DISCORDANCE BETWEEN GREY
SCALE AND DOPPLER FINDINGS
• PSV elevated
• Unilateral
• But no plaque!
– tortuous vessel
– contralateral occlusion/stenosis
INCREASED PSV & NO PLAQUE
TORTUOUS VESSELS
• Velocity increases
around a curve
• Difficult to assign
correct Doppler angle
as direction of blood
flow changes rapidly
INCREASED PSV IN CCA & ICA
• Tortuous vessel
PSV = 269 cm/s
PSVR ~ 2
? 70-95% or 50-69% stenosis
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CONTRALATERAL HI-GRADE
STENOSIS/OCCLUSION
CONTRALATERAL HI-GRADE
STENOSIS/OCCLUSION
•  PSV in CCA and ICA, esp at a stenosis
• Variable, unpredictable
• Use of PSVR may not compensate, but
probably better than using PSV alone
PSV = 269 cm/s
PSVR ~ 2
50% stenosis at most
DISCORDANCE BETWEEN GREY
SCALE AND DOPPLER FINDINGS
Beckett, AJNR: 1990
AbuRahma, J Vasc Surg: 1995
Busuttil, Am J Surg: 1996
TANDEM LESIONS
• Plaque – LOTS!
• But PSV not as elevated as one would expect:
– tandem lesions
– long segment stenosis
– > 95% stenosis
• PSV < expected for a % stenosis in distal lesion
LONG SEGMENT STENOSIS
LONG SEGMENT STENOSIS
• Most atherosclerotic plaques ~ 1 cm in length
• Doppler parameters derived from pts with short
segment plaque
• If plaque extends over more than 2 cm
– PSV will 
– diastolic velocity usu remains high
• Likely due to increased in-flow resistance
– resistance is proportional to length of stenosis
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Spencer and Reid, Stroke: 1979
TIGHT STENOSIS
• If stenosis is > 95%, esp a long segment stenosis
– PSV will 
TIGHT STENOSIS
TIGHT STENOSIS
CLUES TO A TIGHT STENOSIS:
> 95%
“KNOCKING” WAVEFORM
•  diameter of lumen on grey scale and/or color
images
• Reversed / absent diastolic flow proximally
• Low PSV
• Little, reversed, or no diastolic flow
• High resistance waveform pattern
– high resistance waveform
• Tardus parvus waveform distally
– you should always sample as distally as possible
in the ICA
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“KNOCKING” WAVEFORM
• Occurs proximal to an occlusion or high grade
stenosis
– atherosclerosis
– dissection
– vasospasm
– increased ICP
TARDUS PARVUS WAVEFORM
• Delayed systolic upstroke
• Decreased PSV
• Rounded systolic peak
• More pronounced the closer one samples to the
obstructing lesion
TARDUS PARVUS WAVEFORM
TIGHT STENOSIS
• Occurs distal to a high grade stenosis
• The more distal to the stenosis, the more
pronounced
• Pattern of distribution can help localize stenosis
Proximal CCA
HIGH GRADE ICA STENOSIS vs
OCCLUSION
• Important to differentiate
• Occlusion not operable
• In fact, can’t operate on all “string signs”
– esp if extends past skull base
– sometimes ligated
– depends on status of distal circulation, length
– may require evaluation w/ CTA or MRA
Distal ICA
HIGH GRADE ICA STENOSIS vs
OCCLUSION
• No imaging modality is perfect
• US may give F+ Dx of occlusion due to low
volume slow flow
– >15% F+ rate
– confirm with angio or CTA
• US may not be able to differentiate focal TIGHT
ICA stenosis w/ collapse of distal lumen from
multifocal dxs or long segment diffuse
narrowing
El-Saden, Radiology: 2001
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HIGH GRADE ICA STENOSIS vs
OCCLUSION
• Occ US will demonstrate flow not seen on
angio due to delayed filling of distal vessel
20 slow flow
El-Saden, Radiology: 2001
OCCLUSION
• Grey scale:
– hypoechoic, intraluminal echoes
• Color Doppler:
– one vessel at bifurcation
• Spectral Doppler:
– no flow
– high resistance waveform in CCA
– internalization of ECA (↑ diastolic flow)
HIGH GRADE ICA STENOSIS
HIGH GRADE ICA STENOSIS vs
OCCLUSION
• Optimize color Doppler settings
– ↑ gain
– ↓ PRF or scale
– small, straight color box
– ↓ wall filter
• Use power and spectral Doppler before you
refer to MRA or CTA!
• PSV may be lower than you expect
HIGH GRADE ICA STENOSIS vs
OCCLUSION
HIGH GRADE ICA STENOSIS vs
OCCLUSION
• Occluded Rt ICA
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HIGH GRADE ICA STENOSIS vs
OCCLUSION
US suggests occlusion
HIGH GRADE ICA STENOSIS vs
OCCLUSION
• CTA demonstrates “string sign”
False Positive US
OCCLUSION
• PITFALLS:
– large branch of ECA serving as collateral mistaken
for ICA
– filling of 1 cm “stump” of proximal ICA
– “thump” sign
– Doppler US not always adequately sensitive for
detection of slow flow
• Always confirm suspected ICA occlusion with
angio or CTA
CONCLUSIONS
• PSV is the single most important Doppler
criterion
– post-stenotic turbulence should be observed
• PSVR useful esp if PSV in CCA > 100 cm/s,
< 60 cm/s, or if contralateral high grade
stenosis/occlusion
CONCLUSIONS
• To avoid pitfalls:
– consider all spectral Doppler parameters
– integrate grey scale, color Doppler and spectral
Doppler findings
– explain discrepancies
– be attentive to individual physiology
CONCLUSIONS
• Refer to CTA or Angio
– when unexplained discordance btwn spectral
Doppler criteria or btwn spectral Doppler criteria
and grey scale or color flow findings
– if spectral Doppler criteria close to discriminatory
values
– to confirm ICA occlusion vs string sign
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